This invention relates to an apatite-containing film having photocatalytic activity, a light-transmitting materials comprising said film, and a process for producing said film.
Recently, photocatalysts are intensively investigated with a view to imparting antifouling, odor-masking and antibacterial properties to building materials (e.g. plate glass and tiles), electronic equipment (e.g. personal computers and cell phones), consumer electric appliances (e.g. refrigerators and air cleaners), interior furnishings (e.g. curtains), household goods, medical tools, and the like. (see K. Hashimoto and A. Fujishima, “Sanka chitan hikari shokubai no subete—kohkin, bouo, kuhki joka no tameni—(All About Titanium Oxide Photocatalysts—For Antibacterial, Antifouling and Air Cleaning Purposes—)”, CMC, 1988; and K. Hashimoto, “Saishin hikari shokubai gijutsu to jitsuyoka senryaku (Latest Photocatalysis Technolgoy and Its Implementation Strategies)”, BKC, 2002). Products containing photocatalysts exhibit the desired characteristics in themselves. Furthermore, they can decompose contaminants in the surrounding environment, thus contributing to environmental clean-up.
Consider, for example, personal computers and cell phones. A problem with them is that lipids, proteins and carbohydrates from hands, tobacco tar, contaminants in the atmosphere, viruses, bacteria, fungi, and the like are likely to adhere to the keyboard, mouse, buttons and casing, often impairing the appearance of the equipment. In particular, a transparent cover of a display device as a part of such equipment has a strong need for antifouling property in order to retain their light-transmitting properties. Similarly, building materials for daylight have a strong need for antifouling property in order to retain their light-transmitting properties. Attempts are therefore being made to impart antifouling and antibacterial properties by adding photocatalytic materials to those components and materials.
The photocatalytic reaction comprises a stage where the reactant is adsorbed on the catalyst; and a stage where electrons and/or holes, which is generated by light absorption of the catalyst, move to adsorbed species that then undergo reaction. Conventionally, from the viewpoint of electron and/or hole generation by absorption of light, semiconductor materials have drawn researcher's attention as photocatalytic materials. A representative material is titanium dioxide (TiO2).
When a semiconductor material absorbs photons having a larger energy than its band gap, electrons in the valance band are excited to the conduction band, leaving holes in the valence band. If the generated electrons and holes move to adsorbed species, the absorbed species are reduced and oxidized, respectively. In the case of titanium dioxide, adsorbed water is oxidized to generate hydroxyl radicals (.OH) whereas adsorbed oxygen is reduced to generate superoxide anions (.O2−). These radicals and anions in turn react with other adsorbed species and contribute to their oxidation and decomposition.
Titanium dioxide exhibits the desired characteristics in terms of electron and hole generation. However, it also has the following problems. First, among the substances that are needed to be removed by photocatalytic reaction are those which are not easily adsorbed on titanium dioxide. It is often difficult to fully remove such substances by titanium dioxide. This is why there has been a need for photocatalytic materials having high adsorbing capability.
As an additional problem, even though each of a substrate and a titanium dioxide film deposited on the substrate has a good light-transmitting property by itself, combination of the substrate and the deposited film may deteriorate transparency of the material as a whole. Such deterioration is caused by a large refractive index mismatch between the titanium dioxide film and the substrate. In the presence of such a large refractive index mismatch, light reflected on the surface of the TiO2 film may disadvantageously interfere with light passing through the film to be reflected on the interface as well as light of multiple reflection, thereby producing interference fringes.
Exemplified products that are required to have good light-transmission include a protective cover of a display device and a transparent building material. In most cases, these products employ glass as the substrate. The refractive index of titanium dioxide is about three times that of glass. Therefore, in order to suppress the occurrence of interference fringes in those products, it is also desired to develop photocatalysts having refractive indices close to that of glass.
If the areas to be provided with photocatalytic activity are large, photocatalytic materials must be formed in film. It is known to form films of photocatalytic materials by physical deposition techniques such as sputtering and laser ablation, but these techniques require forming films under vacuum. They also involve difficulty in forming uniform, large-area films. A further problem is that the performance of the photocatalytic materials decreases during the process of ion collision or laser irradiation. Another known method comprises the steps of preparing a photocatalytic material, dividing it into particles, and applying them together with a binder to form a film. However, this method suffers a problem of lowered photocatalytic activity because the binder blocks the contact between the photocatalyst and the atmosphere. Hence, it is also desired to develop a simple method for preparing large-area films having good photocatalytic activity.
A Ti-containing calcium hydroxyapatite has been reported as a photocatalytic material that satisfies the requirements on adsorbing capability and refractive index (see JP 2000-327315 A). However, the apatite is rarely soluble and has great tendency to precipitate, thus presenting difficulty in controlling the reaction of the starting materials and the thickness of the formed film in the wet process. Therefore, no simple method has been reported for preparation of apatite films having photocatalytic activity and transparency.